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>Video for Linux Two API Specification: Revision 0.24</TH
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>Chapter 3. Input/Output</TD
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><H1
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><A
NAME="FIELD-ORDER"
>3.6. Field Order</A
></H1
><P
>We have to distinguish between progressive and interlaced
video. Progressive video transmits all lines of a video image
sequentially. Interlaced video divides an image into two fields,
containing only the odd and even lines of the image, respectively.
Alternating the so called odd and even field are transmitted, and due
to a small delay between fields a cathode ray TV displays the lines
interleaved, yielding the original frame. This curious technique was
invented because at refresh rates similar to film the image would
fade out too quickly. Transmitting fields reduces the flicker without
the necessity of doubling the frame rate and with it the bandwidth
required for each channel.</P
><P
>It is important to understand a video camera does not expose
one frame at a time, merely transmitting the frames separated into
fields. The fields are in fact captured at two different instances in
time. An object on screen may well move between one field and the
next. For applications analysing motion it is of paramount importance
to recognize which field of a frame is older, the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>temporal
order</I
></SPAN
>.</P
><P
>When the driver provides or accepts images field by field
rather than interleaved, it is also important applications understand
how the fields combine to frames. We distinguish between top and
bottom fields, the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>spatial order</I
></SPAN
>: The first line
of the top field is the first line of an interlaced frame, the first
line of the bottom field is the second line of that frame.</P
><P
>However because fields were captured one after the other,
arguing whether a frame commences with the top or bottom field is
pointless. Any two successive top and bottom, or bottom and top fields
yield a valid frame. Only when the source was progressive to begin
with, e.&nbsp;g. when transferring film to video, two fields may come from
the same frame, creating a natural order.</P
><P
>Counter to intuition the top field is not necessarily the
older field. Whether the older field contains the top or bottom lines
is a convention determined by the video standard. Hence the
distinction between temporal and spatial order of fields. The diagrams
below should make this clearer.</P
><P
>All video capture and output devices must report the current
field order. Some drivers may permit the selection of a different
order, to this end applications initialize the
<CODE
CLASS="STRUCTFIELD"
>field</CODE
> field of struct&nbsp;<A
HREF="c2030.htm#V4L2-PIX-FORMAT"
>v4l2_pix_format</A
> before
calling the <A
HREF="r10944.htm"
><CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
></A
> ioctl. If this is not desired it should
have the value <CODE
CLASS="CONSTANT"
>V4L2_FIELD_ANY</CODE
> (0).</P
><DIV
CLASS="TABLE"
><A
NAME="V4L2-FIELD"
></A
><P
><B
>Table 3-8. enum v4l2_field</B
></P
><TABLE
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WIDTH="38%"
TITLE="C1"><COL
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TITLE="C2"><COL
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TITLE="C3"><TBODY
VALIGN="TOP"
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_ANY</CODE
></TD
><TD
>0</TD
><TD
>Applications request this field order when any
one of the <CODE
CLASS="CONSTANT"
>V4L2_FIELD_NONE</CODE
>,
<CODE
CLASS="CONSTANT"
>V4L2_FIELD_TOP</CODE
>,
<CODE
CLASS="CONSTANT"
>V4L2_FIELD_BOTTOM</CODE
>, or
<CODE
CLASS="CONSTANT"
>V4L2_FIELD_INTERLACED</CODE
> formats is acceptable.
Drivers choose depending on hardware capabilities or e.&nbsp;g. the
requested image size, and return the actual field order. struct&nbsp;<A
HREF="x5953.htm#V4L2-BUFFER"
>v4l2_buffer</A
>
<CODE
CLASS="STRUCTFIELD"
>field</CODE
> can never be
<CODE
CLASS="CONSTANT"
>V4L2_FIELD_ANY</CODE
>.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_NONE</CODE
></TD
><TD
>1</TD
><TD
>Images are in progressive format, not interlaced.
The driver may also indicate this order when it cannot distinguish
between <CODE
CLASS="CONSTANT"
>V4L2_FIELD_TOP</CODE
> and
<CODE
CLASS="CONSTANT"
>V4L2_FIELD_BOTTOM</CODE
>.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_TOP</CODE
></TD
><TD
>2</TD
><TD
>Images consist of the top field only.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_BOTTOM</CODE
></TD
><TD
>3</TD
><TD
>Images consist of the bottom field only.
Applications may wish to prevent a device from capturing interlaced
images because they will have "comb" or "feathering" artefacts around
moving objects.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_INTERLACED</CODE
></TD
><TD
>4</TD
><TD
>Images contain both fields, interleaved line by
line. The temporal order of the fields (whether the top or bottom
field is first transmitted) depends on the current video standard.
M/NTSC transmits the bottom field first, all other standards the top
field first.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_SEQ_TB</CODE
></TD
><TD
>5</TD
><TD
>Images contain both fields, the top field lines
are stored first in memory, immediately followed by the bottom field
lines. Fields are always stored in temporal order, the older one first
in memory. Image sizes refer to the frame, not fields.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_SEQ_BT</CODE
></TD
><TD
>6</TD
><TD
>Images contain both fields, the bottom field
lines are stored first in memory, immediately followed by the top
field lines. Fields are always stored in temporal order, the older one
first in memory. Image sizes refer to the frame, not fields.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_ALTERNATE</CODE
></TD
><TD
>7</TD
><TD
>The two fields of a frame are passed in separate
buffers, in temporal order, i.&nbsp;e. the older one first. To indicate the field
parity (whether the current field is a top or bottom field) the driver
or application, depending on data direction, must set struct&nbsp;<A
HREF="x5953.htm#V4L2-BUFFER"
>v4l2_buffer</A
>
<CODE
CLASS="STRUCTFIELD"
>field</CODE
> to
<CODE
CLASS="CONSTANT"
>V4L2_FIELD_TOP</CODE
> or
<CODE
CLASS="CONSTANT"
>V4L2_FIELD_BOTTOM</CODE
>. Any two successive fields pair
to build a frame. If fields are successive, without any dropped fields
between them (fields can drop individually), can be determined from
the struct&nbsp;<A
HREF="x5953.htm#V4L2-BUFFER"
>v4l2_buffer</A
> <CODE
CLASS="STRUCTFIELD"
>sequence</CODE
> field. Image
sizes refer to the frame, not fields. This format cannot be selected
when using the read/write I/O method.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_INTERLACED_TB</CODE
></TD
><TD
>8</TD
><TD
>Images contain both fields, interleaved line by
line, top field first. The top field is transmitted first.</TD
></TR
><TR
><TD
><CODE
CLASS="CONSTANT"
>V4L2_FIELD_INTERLACED_BT</CODE
></TD
><TD
>9</TD
><TD
>Images contain both fields, interleaved line by
line, top field first. The bottom field is transmitted first.</TD
></TR
></TBODY
></TABLE
></DIV
><DIV
CLASS="FIGURE"
><A
NAME="FIELDSEQ-TB"
></A
><P
><B
>Figure 3-1. Field Order, Top Field First Transmitted</B
></P
><DIV
CLASS="MEDIAOBJECT"
><P
><IMG
SRC="fieldseq_tb.gif"></P
></DIV
></DIV
><DIV
CLASS="FIGURE"
><A
NAME="FIELDSEQ-BT"
></A
><P
><B
>Figure 3-2. Field Order, Bottom Field First Transmitted</B
></P
><DIV
CLASS="MEDIAOBJECT"
><P
><IMG
SRC="fieldseq_bt.gif"></P
></DIV
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